Diet Study Questions Low-Fat Conventional Wisdom

The post-Labor Day news headlines are awash with the results of what appears to be an important new diet and weight loss study. What’s more, the results seem to confirm the too-many-carbs-is-the problem hypothesis made popular by Gary Taubes and many others.

In a relatively large, randomized trial conducted at Tulane University, researchers put obese but healthy volunteers on a 12-month diet that was either “low fat” or “low carb.” The low-carb dieters lost an average of about 8 pounds more than the low-fat eaters, even though both started at about the same weight and consumed similar calories per day.

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More surprisingly, the low-carb dieters enjoyed greater improvements in heart-health measures such as total cholesterol, HDL cholesterol, and c-reactive protein than the low-fat subjects. In other words, their relatively high consumption of fats (40.7 percent of all calories, including nearly double the saturated fat intake recommended by the American Heart Association) didn’t ruin their blood lipid profiles. To the contrary, the high-fat diet improved their blood fat levels, apparently turning upside down nearly 40 years of U.S. dietary recommendations about fat consumption and heart health.

The study did not last long enough to measure any mortality outcomes of the subjects. But the low-carb eaters enjoyed improved scores on the well-known Framingham Heart Risk Score.

The study turned up many other interesting findings that haven’t been mentioned in most media reports.

For example, all the dieters "should" lost roughly 55 pounds during the 12 months, given their daily shortfall of about 500 calories per day from their previous diets. The study didn’t measure activity levels, but subjects were told to continue in their normal habits. So the weight loss didn’t come easily, and didn’t follow a simple calories-in/calories-out model.

Also, the trial showed no evidence of the glucose/insulin link proposed by Taubes and other diet writers as the mechanism behind the success of low-carb diets. Indeed, the low-fat/higher-carb eaters appeared to gain slightly more glucose/insulin control than the low-carb dieters.

So it’s tempting to say that the Tulane University trial burst two balloons at once: First, that high-fat diets are bad for heart health; and second that high-carb diets juice the glucose-insulin-fat deposition system.

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It’s likely that the strictest proponents of low-fat and low-carb will both be displeased with the new study. The low-fat eaters received 30 percent of their calories from fat. The low-carb eaters received 34 percent of their calories from carbs. Ardent supporters of low-fat and low-carb diets would both like to see these percents much lower.

In one strange outcome of the study, the low-fat dieters decreased their absolute carbohydrate consumption from study beginning to end, due to their lower overall calorie intake.

Here are the percents of calories from fat and carbs for the two groups at the study’s outset and at the 12 month mark:

For runners who may want to try a lower-carb diet but still get sufficient high-octane fuel (the body “prefers” carbohydrates for high-intensity exercise like running), the best bet is probably to cut back carbs at all meals except the one before your running workout. Before a marathon, several days of carb-loading will likely pay off, but you don’t necessarily need high carbs all the time during your regular training.

In the Tulane study, researchers hoped the low-carb group would consume just 160 calories a day in carbs. They couldn’t manage that low a target; they came in at closer to 350 carb calories a day. Carbs are everywhere in our modern diet. For now, the best advice is to eat modest amounts of high-quality carbs, i.e., those that aren’t heavily processed and laden with sugars.

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